1. Field of the Invention
This invention relates to a mold clamping mechanism in a molding machine such as an injection molding machine.
2. Description of the Related Art
FIG. 11 is a schematic view for use in an illustration of a conventional mold clamping mechanism of a molding machine such as an injection molding machine. In the conventional mold clamping mechanism of the molding machine such as the injection molding machine, a fixed platen 2 is mounted in an upright position to a base frame 1 of the molding machine. The fixed platen 2 is connected to a rear platen 3 with tie bars 5, typically, four pieces of tie bars, and a movable platen 4 having through holes permitting these tie bars 5 to pass through is placed between the fixed platen 2 and the rear platen 3. With a moving-side mold 17a mounted to the movable platen 4, and with a fixed-side mold 17b mounted to the fixed platen 2, the conventional mold clamping mechanism drives the movable platen 4 with a drive source such as a motor and a hydraulic cylinder to effect clamping of the molds 17a and 17b in such a manner as to move the movable platen 4 with the tie bars 5 as guides.
The conventional mold clamping mechanism uses a slide bearing 15 for a sliding surface of the movable platen 4 with each tie bar 5. The through holes of the movable platen have slide bearing bushes 15a, 15b to ensure that each tie bar 5 passes through the bushes 15a, 15b. 
The above conventional mold clamping mechanism requires the slide bearing at a sliding part of the movable platen with each tie bar, leading to an increase in clearance or play between an inside diameter of each slide bearing bush and an outside diameter of each tie bar. Thus, tilting of the movable platen is caused easily, resulting in degradation of the degree of parallelism between a surface of the moving-side mold mounted to the movable platen and a surface of the fixed-side mold mounted to the fixed platen.
In Japanese Patent Application Laid-open No. 2001-260196, for instance, there is disclosed a mold clamping mechanism of a molding machine, in which a movable platen has tie bar insertion holes each having ball spline nuts, while a sliding part of each tie bar on which at least the movable platen slides is given in the form of a ball spline shaft to ensure that prevention of tilting of the movable platen is provided.
FIG. 12 is a schematic view of the other conventional mold clamping mechanism described in the above Patent document. FIG. 13 is a view taken in the direction of an arrow A in FIG. 12. The movable platen 4 is moved along the tie bars 5 with a ball spline mechanism composed of the ball spline nuts 6a, 6b provided in each tie bar insertion hole and a ball spline shaft 14 provided for each tie bar 5.
The ball spline shaft of the above conventional mold clamping mechanism suffers a force inclusive of a mold clamping force, so that a diameter of the ball spline shaft needs to be sized equal to or larger than a diameter of each tie bar shaft. Thus, in application of the above arrangement to a large-sized molding machine, the above conventional mold clamping mechanism requires a ball spline shaft having a large diameter more than necessary, thereby providing disadvantages of causing an increase in cost, and besides, of being difficult to manufacture.